Literature review: Impact of Chilean needle grass ... - Weeds Australia

esult in a “short-term flush” of N mineralisation in the soil, and suggested that T. triandra is uniquely able to take advantage of
this resource.
The critical factor for survival and resprouting of vascular plants of frequently burned grasslands is the degree to which buds are
protected from fire damage (Overbeck and Pfadenhauer 2007). In general in temperate south-eastern Australian grasslands fire
promotes vigorous resprouting by native perennials, but generally very little perennial seedling recruitment: fire enhances the
vigour and flowering of many perennial herbs but results in little change in plant species composition (Lunt and Morgan 2002).
However buring can lead to major increase in the abundance of annual exotic species on long-unburnt sites (Lunt 1990c). T.
triandra grassland fires are relatively cool, and when occurring in summer and early autumn have little negative effect on the
flora, which survives with maximal underground carbohydrate storage, buried buds and buried seed, although seeds on the soil
surface are destroyed (Lunt and Morgan 2002). The majority of intact T. triandra grasslands on the Victorian volcanic plains are
burnt at intervals of 1-5 years (Morgan 1998d) a frequency recommended by McDougall (1989). Healthy T. triandra swards are
usually maintained with fires at this frequency and no grazing (Lunt and Morgan 2000). The time of burning (late summer or
autumn) is probably not critical for the health of T. triandra populations, but fire greatly reduces the amount of T. triandra seed
produced in the following autumn (McDougall 1989). Reduced fecundity in the short term is compensated for by increased plant
surival and vigour. Morgan (1997) predicted that reducing the fire frequency from 1-2 years to 5 or more years would
substantially alter the dynamics of the population if seedling recruitment was reduced and established plants were incapable of
adjusting.
Most of our knowledge relates to the grasslands in which T. triandra is dominant and highly productive, systems that naturally
promote fire. Much less is known about fire effects in grasslands not dominated by T. triandra. Poa spp. “appear to be able to
maintain their dominance without disturbance by burning or other forms of biomass removal” while Austrodanthonia and
Austrostipa grasslands accumulate little biomass and are not thought to need regular biomass reduction to maintain plant
diversity (Lunt and Morgan 2002 p. 183). Little seems to be known about fire effects in the Riverine Plains grasslands.
Themeda triandra biomass accumulation and senescence dieback
The dependence of T. triandra grasslands on fire is almost universally recognised, despite the very limited understanding of their
evolutionary origin and palaeoecology, particularly in relation to ancient grazing regimes. They appear to be similar to temperate
fire-adapted grasslands with C 4 dominants worldwide, where the C 4 species promote their own dominance by building high
levels of biomass and thus promoting fire. T. triandra stands that are not burnt, or otherwise biomass-reduced, gradually develop
massive quantities of dead leaves and litter, and failure to remove this biomass can cause tiller and plant senescence, attributed to
“self-shading” (Lunt and Morgan 2000). Major T. triandra mortality occurred at Derrimut and Laverton North grasslands when
fire frequency exceeded 5 y, and when fire was finally used, plant and tiller densities were much lower than in regularly burnt
grassland (Morgan and Lunt 1999, Lunt and Morgan 1999a 1999c). Mass dieback of T. triandra resulting from the absence of
fire or other biomass reduction has been described as “grassland collapse” (C. Hocking pers. comm.).
According to Lunt and Morgan (1998a p.70) “the grassland becomes increasingly choked up with dead grass material, above
which the tussocks form a thin mantle of new, green growth”, and eventually living tillers can “no longer poke up through the
dead grass to reach the sunlight, causing the tussocks to die”. Supposedly, there is “insufficient light penetrating through the
canopy of old foliage to the young tillers to enable them to photosynthesise sufficient energy” (Lunt et al. 1998). T. triandra
grassland not burnt for greater than 6 years is now thought to senesce in this way (Morgan and Lunt 1999, Lunt and Morgan
2002), rather than reach a steady state (Lunt and Morgan 2002). Low soil fertility and moisture levels may sometimes prevent
such senescence (Kirkpatrick et al. 1995), so on less productive sites T. triandra senescence requires considerably longer periods
or may never occur. O’Shea (2005 p. 161) stated that on productive sites, the phenomenon requires 10-11 years: “the canopy
collapses upon itself and forms a thick layer of dead thatch over the soil surface, allowing only minimal seedling recruitment and
preventing new tiller initiation”. According to Muyt (2005 p. 3), the dense T. triandra thatch “undermines the growth of [the
grass] itself; plants become increasingly brittle and subject to collapse”. After burning T. triandra usually regains high cover
quickly, returning to pre-fire biomass levels in 2-4 years (Morgan 1994, McDougall and Morgan 2005) and can form a complete
dense canopy in as little as 3 years. Death of the dominant grass results in a major nutrient pulse in the soil, resulting primarily
from decay of T. triandra crowns and roots, and this enables invasion by exotic plants (Wijesuriya 1999, Wijesuriya and
Hocking 1999). Weedy exotics are now generally pervasive in these systems, so what would happen in the absence of exotic
seed sources after T. triandra die-off is not clear.
The senescence dieback phenomenon has been widely reported worldwide for other dominant temperate and subtropical C 4
tussock grasses adpated to fire (Bond et al. 2008). Although often described under different rubricks (e.g “detritus accumulation”
– Knapp and Seastedt 1986) the effect is the same: accumulation of standing dead litter shades out and kills the shade-intolerant
new tillers (Knapp and Seastedt 1986; Everson et al. 1988, Uys et al. 2004). Fire frequency is thus one of the most important
determinants of the grass species composition in C 4 grasslands: “fire-dependent grass species, typically members of the
Andropogoneae ... are fire-dependent in the sense that they decrease in, or disappear from, a sward in the absence of frequent
burning” (Bond et al. 2008 p. 1747). Lunt and Morgan (1999a) reported a 70% decrease in T. triandra tussock density and a
58% decrease in live tiller density in rarely burnt areas. Similar changes are indicated by Uys et al. (2004) who found that the
cover of T. triandra in a mesic (735 mm per annum) South African grassland decreased from c. 70% when annually burnt to <
5% after fire exclusion for 4 or more years, and the plant effectively disappeared from mesic and montane sites that were
unburnt..
In southern Brazilian mixed C 3 and C 4 tussock grasslands dominated by C 4 grasses, Overbeck and Pfadenhauer (2007 p. 35)
observed that “without periodic removal of biomass ... shading by dead [grass] biomass inhibits survival and tillering and higher
humidity under the litter may cause death and decay of underground plant parts within a few years”. Post-fire effects that
increase the vigour of tussocks have been recorded for a number of dominant species in other parts of the world, including
increased photosynthetic activity, growth rates and sexual reproduction (Overbeck and Pfadenhauer 2007).
The T. triandra senescence phenomenon may be compared with ‘normal’ grass senescence, which occurs in all Poaceae in
response to drought (Norton et al. 2008) and is a mechanism to reduce plant mortality from water stress, by the gradual
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